The invention relates to an arrangement comprising a carry-save adder for bit-parallel addition of binary numbers in two's complement.
A carry-save adder of this type is known from the book Computer Arithmethic by K. Hwang, John Wiley & Sons, New York, 1979, pp. 98-103, particularly, FIG. 4.2. Every first adder has three inputs which are connected to equivalent bits of three binary numbers which are to be added to one another. The sum outputs of the first adders are connected to first inputs of the adder means and the carry outputs of the first adders (with the exception of the most significant first adder) are connected to second inputs of the adder means. A sum word appears at the outputs of the latter as the result of the addition. In contrast to an adder arrangement having ripple carry (carry-propagate principle), the carries of all the first adders are simultaneously formed by addition of three binary numbers, and are available as a carry word for addition in the adder means, being available at the same time as the intermediate sum word produced by the first adders. An adder constructed in this fashion works on what is referred to as the carry-save principle.
When binary numbers are externally supplied to the third inputs of the first adders, and a first intermediate result achieved by a preceding addition is supplied to the first two inputs, such first intermediate results being composed of an intermediate sum word supplied to the first inputs and of a carry word supplied to the second inputs, then a second intermediate result formed by addition of the first intermediate result and this number is produced at the outputs of the first adders.
A continuing formation of constantly new intermediate results which are obtained, given a continuous supply of further binary numbers via the third inputs, is referred to as an accumulation of these numbers. An overflow of the sum word can then occur, in that the allowable adder content which is dependent on the prescribed plurality of first adders is upwardly (or downwardly) exceeded for positive (or negative) contents. With a carry-save adder employed in recursive circuits, such an overflow frequently means that the adder contact begins to periodically change between two limit values, whereby the constantly changing sum word at the output of the adder means corresponds to an analog oscillation. Such behavior of an adder in a recursive circuit is described in Proc. of the IEEE, Vol. 63, No. 4, April 1975, pp. 633-648, cf., in particular, FIGS. 5 and 6 and the appertaining text of page 636.